【单选题】

We find that bright children are rarely held back by mixed-ability teaching. On the con-trary, both their knowledge and experience are enricheD、We feel that there are many dis-advantages in streaming (按智力划分) pupils. It does not take into account the fact that children develop at different rates. It can have a bad effect on both the bright and the not-so-bright chilD、After all, it can be quite discouraging to be at the bottom of the top grade!

Besides, it is rather unreal to grade pupils just according to their intellectual ability. This is only one aspect of their total personality. (76) We are concerned to develop the abilities of all our pupils to the full, not just their academic ability. We also value personal qualities and social skills, and we find that mixed-ability teaching contributes to all these aspects of learning.
In our classrooms, we work in various ways. The pupils often work in groups: this gives them the opportunity to learn to co-operate, to share, and to develop leadership skills. They also learn how to cope with personal problems as well as learning how to think, to make decisions, to analyze and evaluate, and to communicate effectively. The pupils learn from each other as well as from the teacher.
Sometimes the pupils work in pairs; sometimes they work on individual tasks and as-signments, and they can do this at their own speeD、They also have some formal class teaching when this is appropriate. We encourage our pupils to use the library, and we teach them the skills they need in order to do this efficiently.An advanced pupil can do ad-vanced work: it does not matter what age the child is. We expect our pupils to do their best, not their least, and we give them every encouragement to attain this goal.
By "held back" (Line 1) the author means "______. "
A、made to remain in the same class
B．forced to study in the lower class
C．drawn to their studies
D．prevented from advancing